Declan would go perfectly with your sibling set. I would nix Harpur and Slaine. Harpur will have spelling issues and Slaine looks like a trendy smoosh even though it is legit. I would worry about it being confused with the word 'slain'. Depending on where you live, I don't think it is as recognizable as Saoirse and Niamh, so there could be huge pronunciation issues. From your list Rory fits best, but I think it'd be better on a boy. Finn works also.